IB Chemistry SL 2.2.6 Molecular Polarity

Practise deciding molecular polarity in two stages: establish which bonds are polar, then use three-dimensional geometry to determine whether their dipoles cancel. Symmetry, lone…

Exam points

identify polar bonds before using molecular geometry to combine their dipole vectors
explain dipole cancellation in a symmetrical molecule or a net dipole in an asymmetrical one
compare isomers or related molecules by symmetry, shape and resulting molecular polarity

[Maximum number: 2]

The element boron has two naturally occurring isotopes, ${ }^{10} \mathrm{~B}$ and ${ }^{11} \mathrm{~B}$ .

Predict and explain whether the molecules $\mathrm{NH}_{3}$ and $\mathrm{BF}_{3}$ are polar molecules.

$\mathrm{BF}_{3}$ not polar as no net dipole moment / BF bond polarities cancel each other out / symmetrical distribution of charge;
$\mathrm{NH}_{3}$ polar as net dipole moment present / NH bond polarities do not cancel each other out / unsymmetrical distribution of charge;

Marking guidance:

Accept suitable diagram showing dipole moments.
Do not accept electronegativities cancel out.

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2.2.6—Molecular polarity question 1